Fertilization &implantation

Dr: Azza Zaki

Fertilization  

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Definition: It is the fusion of the male & female gametes (union of a sperm and an ovum) to form zygote. Normal site: Ampulla or lateral third of the uterine (fallopian) tube. Time: Within 24 hours from ovulation. Dr: Azza Zaki

Dr: Azza Zaki

Steps of fertilization: 1-Transport of gametes:  Ovum: during ovulation the fimbriated end of the uterine tube becomes closely applied to the ovary.  Sweeping action of tubal fimbriae carries the oocyte into the uterine tube.  The oocyte then passes into the ampulla of the tube by beating of the cilia and peristaltic contraction.  Sperm: of the 200-300 millions sperms deposited in the vagina, only 300-500 Dr: Azza Zaki reach the site of

Ii. Sperm Conditioning Sperms deposited in the female genital tract must undergo two changes before they can fertilize an ovum: 1. Capacitation:  Sperms cannot fertilize oocytes when they are newly ejaculated.  The process of capacitation takes 5-7 hours.  Capacitated sperms are more active.  Location: capacitation occurs in the uterus and oviducts.  The acrosomal reaction cannot occur until capacitation has occurred.  The glycoprotein coat removed from the cell membrane covering the acrosomal cap. Dr: Azza Zaki

2. Acrosome reaction:  Fusion occurs between the outer acrosomal membrane and the cell membrane of the corona radiata with the release of acrosomal enzymes (acrosin). a) Hyaluronidase results in digestion of the corona radiata. b) Trypsin-like substance & zona lysin lead to penetration of the zona pellucida.

During fertilization, the sperm must penetrate: A-the corona radiata. B-the zona pellucida. C.the oocyte cell membrane.

Dr: Azza Zaki

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Phases of fertilization: Phase 1: penetration of corona radiata: many sperms penetrate the corona radiata with the aid of acrosomal enzymes (hyaluronidase) Phase 2: penetration of zona pellucida: Only one sperm penetrates the zona and once the sperm isDr: Azza Zaki

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Phase 3: fusion of the oocyte and sperm cell membranes. After fertilization, as soon as the sperm has entered the oocyte: a) the oocyte finishes its 2nd meiotic division & forms the female pronucleus; b) the zona pellucida becomes impermeable to other sperms;& c) The head of the sperm separates from the tail, swells & forms the male pronucleus.

The female pronucleus comes into close contact with male pronucleus.  The nuclear membranes of the 2 pronuclei disappear and their chromosomes become around the equator Dr: Azza Zaki thus the zygote is 

Results Of Fertilization

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Restoration of the diploid number of chromosome (46). Determination of the sex of the embryo. An Xcarrying sperm produces a female embryo, and a Y- carrying sperm produces a male embryo. Completion of the 2nd maturation (meiotic) division with formation of the mature ovum and 2nd polar body. Initiation of cleavage or cell division of the zygote . The zona & cortical reactions occur to prevent fertilization of the same ovum by more than one sperm. Dr: Azza Zaki

InVitro Fertilization (IVF)  A method of treatment of 

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infertility, if there is occlusion of fallopian tube. The female is given gonadotropin to stimulate multiple ovulations. The ova are collected with aspirator during laparoscopy & are put in a glass plate. Sperms collected from the husband are added immediately. After fertilization, the eggs are monitored to 8- cell stage then placed in the uterus to develop to term. Dr: Azza Zaki

CLEAVAGE  

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Definition: Is a series of mitotic divisions of the zygote results in an increase in cells number without increase in total size. Site: uterine tube. Dr: Azza Zaki

Stages Of Cleavage 

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The zygote divides by mitosis into twocell stage. Each cell is named blastomere. Repeated mitosis giving 4 cells, 8 cells& so on until a Morula of 12-16 compacted cells is formed after 3 days after fertilization & enters the uterus. The morula is differentiated into an inner cell mass & an outer cell mass. Blastocyst stage: fluid from the uterine cavity accumulates between the blastomeres dividing them into 2 groups, one group of cells in the center (inner cell mass) which will give the embryo so it is called (embryoblast)& 2nd group of cells at the periphery, forming the wall of blastocyst (outer cellDr:mass) Azza Zaki which will

Blastocyst

Consists of: Inner cell mass: 1-embryoblast : large cells on the embryonic pole of the blastocyst. These cells form the embryo (embryo proper)  2-Outer cell mass: trophoblast : small cells forming the wall of blastocyst. They forms the foetal membranes and placenta.3-Blastocoele: a cavity.  The zona pellucida has disappeared, allowing implantation to begin.

Dr: Azza Zaki

Dr: Azza Zaki

Dr: Azza Zaki

Implantation

Definition: It is the embedding of the blastocyst into the compact layer of the endometrium. Time: It begins about the 6th day after fertilization. It is completed by the 11th or Events during the first week of human 12th day. development

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On the 5th day after fertilization, the zona pellucida disappears. The blastocyst increases rapidly in size. On the 6th day, the trophoblast cells covering the embryonic pole adhere to the endometrium & starts to proliferate rapidly.

Dr: Azza Zaki

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It differentiates into

2 layers:  an inner cytotrophoblast  an outer syncytiotrophoblast (cells lose its cell boundaries) - It extends finger-like processes, which secrete proteolytic enzymes that erode the endometrium, creating a hole through which the blastocyst is gradually buried in the endometrium. Dr: Azza Zaki

3. By the 9th -10th day, the hole in the endometrium is closed by a fibrin plug. The endometrium regenerates by the 12th day. Normal site of implantation: Posterior wall of the upper part of the uterus near the fundus, in the midline.  Time: from 7th to 12th day after fertilization. Dr: Azza Zaki

Intrauterine The normal site of implantation is the posterior wall of uterus near Implantation

the fundus.

Dr: Azza Zaki

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Uterus At Time Of Implantation The endometrium is in the secretory phase.

The endometrium is in the secretory phase.

Dr: Azza Zaki

The decidua

It is the endometrium after implantation. After implantation the decidua becomes differentiated into:

Decidua basalis: under the site of implantation. Dr: Azza Zaki Decidua capsularis:

Ectopic pregnancy( abnormal site of implantation)

Dr: Azza Zaki

Abnormal Site of Implantation 1.Intra- uterine: Placenta praevia: Implantation in the lower uterine segment (cervix).  3 types: lateral, marginal and central. Cause severe bleeding before delivery. 2. Extra-uterine: (Ectopic pregnancy): Outside the uterus:  Tubal  ovarian  abdominal. Dr: Azza Zaki

Cervical pregnancy (Placenta Previa) Implantation in the lower  

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uterine segment (cervix). Types of placenta previa: Placenta previa centralis: the placenta completely covers the internal os. Placenta previa marginalis: the margin of the placenta reaches the internal os. Placenta previa lateralis: the margin of the placenta does not reach the internal Dr: Azza Zaki os.

Ectopic Implantation The most common site of ectopic pregnancy is tubal pregnancy.

Dr: Azza Zaki

Dr: Azza Zaki

a) Tubal pregnancy (95%): Implantation in the fallopian tube causing its rupture.  It usually ruptures leading to pain that is commonly confused with appendicitis if on the right side.  Rupture usually occurs in the 2nd month causing internal haemorrhage which endanger life of the mother. b) Ovarian pregnancy c) Abdominal pregnancy: Implantation in the peritoneal cavity (Douglas pouch or mesentery. Dr: Azza Zaki

Dr: Azza Zaki